The invention relates to method and apparatus for quantitative chromatographic analysis of a mixture of weak anions (e.g., organic carboxylates) or weak cations (e.g., ammonia, organic amines) in aqueous sample solutions.
Heretofore, mixtures of organic carboxylic acids or their carboxylate salts have been analyzed by chromatographic separation with a single column of ion exchange resin followed by detection with an electrical conductivity device. If water soluble metal halides are present in the sample, a pre-column of silver from cation exchange resin is used in addition to the chromatographic column to remove excess halide ions by precipitation. A system of this type is described in Small et al U.S. Pat. 3,920,398. This method allows separation and detection of the carboxylic acids or their salts. The above system is subject to a number of drawbacks. Detection limits range from approximately 0.1 parts by per million (ppm) for acids with one to three carbon atoms, approximately one ppm for acids with four to five carbons and approximately 1 to 100 ppm for acids with 8 to 10 carbon atoms. Also, chromatographic resolution of these low molecular weight carboxylic acids is limited by the necessity of using a very low electrically conducting eluent such as water. This latter requirement is necessary due to the use of an electrical conductivity detector; otherwise, high ionic strength eluents would give rise to high conductivity background and further limit detection sensitivity.
Ion chromatography (IC) has been used to determine both organic carboxylic acids and amines. An IC system for separating cations is described by Stevens U.S. Pat. 3,926,559. IC has the general problem that in anion analysis, weak and strong anions (e.g., organic acids and inorganic anions) coelute unresolved making quantitative analysis of these species difficult. This is also true for weak and strong cations (e.g., organic amines and alkali metal ions). Also, multi-charged weak anions and cations are difficult to elute by IC requiring high eluent concentrations. This necessarily increases the regeneration frequency of the suppressor column and, in a practical sense, measurably adds difficulty to the analysis. Also, because the salt of a weak acid must be used as eluent for IC anion analysis, analysis of that particular weak acid in the sample is precluded.